It is known that in the process of producing metallurgical coke for the operation of blast furnaces, coal is heated in order to eliminate most of the volatile components and preserving mostly the carbon structure. Coke is thereby provided with the physical and chemical properties which make it fit for providing energy and burden support in blast furnaces. Volatile matter of coal comprises a number of compounds which are distilled in coke ovens constituting what is known as coke oven gas.
The volume and composition of COG produced in coke ovens depends on the characteristics of the coal utilized. Raw coke oven gas coming from the coke oven battery has the following typical composition: water about 47%; hydrogen 29% to 55%; methane 13% to 25%; nitrogen 5% to 10%; carbon monoxide 3% to 6%; carbon dioxide 2% to 3%; hydrocarbons (ethane, propane etc.) 2% to 1%; and various contaminants such as tar vapors and light oil vapors (aromatics), consisting mainly of benzene, toluene and xylene (these three generally known as BTX); naphthalene; ammonia; hydrogen sulfide; hydrogen cyanide and other impurities.
Raw COG must be cooled, cleaned and treated in a number of chemical processes for separating valuable compounds such as ammonia and other petrochemicals and for removing sulfur, gums and other substances, so that it may be used as a fuel gas at the coke oven battery and elsewhere in the steelmaking plant. In the COG treatment plant, COG is cooled down to condense out water vapor and contaminants and for removing tar aerosols to prevent gas line/equipment fouling. Ammonia is also removed to prevent gas line corrosion, and naphthalene to prevent gas line fouling by condensation. Light oil is separated for recovery and sale of benzene, toluene and xylene, and hydrogen sulfide has to be removed to meet local emissions regulations.
After this treatment, COG typically has the following composition: about 61% hydrogen; about 8% carbon monoxide; about 4% carbon dioxide; about 22% methane; about 1% nitrogen; about 2% water; about 2% of hydrocarbons heavier than methane including ethylene and acetylene; about 5% BTX; and less than about 1% of hydrogen sulfide, tars and naphthalene.
Since coke oven gas has a high calorific value, it is utilized mostly for heating purposes in steel plants, but the chemical values of hydrogen and carbon monoxide can be advantageously utilized for reduction of iron ores to metallic iron for increasing the iron/steel production of steelmaking facilities.
Direct reduction processes may be utilized in the steel industry as an alternative to blast furnaces or to supplement blast furnaces by utilizing sulfur-containing coke oven gas as a way of increasing the metallic iron production. The most common type of reactor where the DRI is produced is a shaft-type moving-bed furnace, having two main sections: a reduction zone where a reducing gas is circulated at a high temperature and through which said reducing gas is recycled in a reduction circuit and a cooling zone located below the reduction zone where the DRI is cooled down to ambient temperatures before being discharged from said reactor by circulating a cooling gas containing also hydrogen and carbon monoxide in a cooling circuit.
Iron-containing particles in the form of pellets, lumps or mixtures thereof are charged to the upper part of a shaft-type reduction reactor and are reduced to metallic iron by contacting said particles with a reducing gas containing hydrogen and carbon monoxide at temperatures above 850° C.
Oxygen is removed from the iron ores by chemical reactions based on hydrogen (H2) and carbon monoxide (CO), for the production of Direct Reduced Iron (DRI) having a high degree of metallization (ratio of metallic iron to total iron content in the DRI).
The overall reduction reactions involved in the process are well known and are represented below:Fe2O3+3H2→2Fe+3H2O  (1)Fe2O3+3CO→2Fe+3CO2  (2)
The hydrogen and carbon monoxide transformed into water and carbon dioxide according to reactions (1) and (2) are separated from the gas stream circulating in the reduction circuit and are substituted by a make-up feed of reducing gas. The reducing gas make-up generally comes from a natural gas reformer, but according to the invention, this make-up gas is withdrawn from the gas circulating through the lower cooling/discharge zone of the reduction reactor. The DRI present in the cooling/discharge zone contributes in removing heavy hydrocarbons, BTX, tars and other undesirable compounds present in the COG, whereby these substances are not present in the reduction circuit and fouling problems in the gas heater and other equipment are avoided.
There have been several proposals for utilizing COG in direct reduction processes, for example U.S. Pat. No. 4,054,444 teaches a direct reduction process wherein methane or a methane-containing gas is introduced to the shaft furnace beneath its reduction zone for increasing the carbon content of the DRI. No specific teaching of utilizing COG is found in this patent. It is however mentioned here in connection with the broader possibility of feeding a methane-containing process gas to the cooling gas loop. The gases injected to the cooling loop in this patent are all allowed to flow upwardly through the furnace from the cooling zone to the reduction zone. This patent does not show the possibility of transferring gas from the cooling loop to the reduction loop by means of an external conduit, therefore the amount of gases which can be fed to the cooling loop are limited to the amount which does not cool down the iron-particles bed in the reduction zone.
U.S. Pat. No. 4,253,867 discloses a method of using “a hydrocarbon-containing gaseous process fuel,” exemplified by COG, for reducing iron ores (wherein a mixture of COG and steam is fed to an intermediate zone located between the reduction zone and the cooling zone of the reduction reactor). Coke oven gas is reformed to hydrogen and carbon monoxide in the reforming zone taking advantage of the catalytic action of the iron and the high temperature of the solid DRI in said reforming zone. This patent does not teach the possibility of feeding all the COG needed for the reduction of iron oxides to the cooling loop nor suggests transferring gas from the cooling loop to the reduction loop through an external conduit.
U.S. Pat. No. 4,270,739 and No. 4,351,513 disclose a direct reduction process where a sulfur-containing “process fuel gas, such as coke oven gas” is desulfurized by the iron-containing particles contained in the reduction reactor by heating and injecting the COG above the reduction zone of the reduction furnace. In the '739 patent, COG is heated in a fired heater before its introduction to the desulfurizing zone and in the '513 patent, COG is heated by heat-exchange with the flue gases of a reformer. These patents do not suggest feeding the COG to the cooling loop and then transferring a major part of said COG from the cooling loop to the reduction loop by means of a conduit external to the reduction furnace (with its increased process control benefit).
Documents cited in this text (including the patents discussed herein), and all documents cited or referenced in the documents cited in this text, are incorporated herein by reference. Documents incorporated by reference into this text or any teachings therein may be used in the practice of this invention.